Prediction Accuracy of Failure Criteria in FE-analyses of Hydroforming Processes

Simulations of tube hydroforming processes require a reliable prediction of the occurrence of plastic instabilities, which are limiting the maximum feasible expansion of the formed tube. To determine the onset of necking, as one of the important instabilities in hydroforming, the use of forming limit diagrams is a well-established practice. This method was initially developed for sheet metal forming processes and is commonly applied also for hydroforming, in spite of the fact that the prediction accuracy depends on the strain path at the analysed position of the formed component. To establish according forming limit diagrams, experimental as well as theoretical techniques exist. Ductile fracture criteria, based on the calculation of forming-history-dependent damage values and considering the development of continuum variables such as stress and strain, are alternative methods of failure prognostication and appealing because of their simplicity. Against the background to obtain an information about the quality in failure prediction of theoretically determined forming limit diagrams and damage-value-based methods for hydroforming component design, an evaluation of , selected examples, of these methods was conducted using simulations with the finite element method. To enable a reliable comparison with experiments, published results of aluminium tube hydroforming were taken where load paths for the forming operation were applied which resulted in a predominantly constant strain ratio at the point of maximum expansion. It was found that the investigated criteria resulted in different prognostication for the onset of plastic instability.